In German open application DE-OS No. 29 28 526 there is described a method of reducing the nitrogen oxide content in the flue gas stream from the boiler of an electricity-generating fossil-fuel power plant wherein, upstream of a reactor capable of transforming the nitrogen oxides to molecular nitrogen as described more fully below, ammonia (NH.sub.3) is added to serve as a reducing agent.
In the reactor, a selective catalytic reduction can be carried out or, if the reactor is maintained at a temperature of 800.degree. to 1000.degree. C., a noncatalytic reduction can be effected, whereby the nitrogen oxides are predominantly transformed to molecular nitrogen which is nontoxic.
During this reaction or subsequent thereto, residual ammonia in the gas stream may react with other components thereof to form precipitate compounds which are scrubbed so that a NH.sub.3 - or NH.sub.4 -containing waste water results.
In other domains in the operation of an electric-generating power plant of this type, NH.sub.3 - or NH.sub.4 -containing waste water may arise as well, e.g. in the wet scrubbing of fly ash or in the flue gas cleaning apparatus apart from the reducing reactor. Such apparatus is a normal part of modern power plant boiler technology for the removal of particulates and for the desulfurization of the stack gases before they are released into the atmosphere. A typical component of such waste waters is ammonium sulfide.
Disposal of the waste water has been a problem heretofore (see EPRI CS-3603, Project 1256-7, Final Report October 1984) since the waste water cannot be disposed of without treatment into open effluent systems. Indeed, the treatments required are comparatively expensive and time-consuming chemical and biological waste water treatments.